A method which has been used to increase the capacity of cellular communication systems is the concept of hierarchical cells wherein a macro-cell layer is underlayed by a layer of typically smaller cells having coverage areas within the coverage area of the macro-cell. In this way, smaller cells, known as micro-cells or pico-cells (or even femto-cells), are located within larger macro-cells. The micro-cells and pico-cells have much smaller coverage thereby allowing a much closer reuse of resources. Frequently, the macro-cells are used to provide coverage over a large area, and micro-cells and pico-cells are used to provide additional capacity in e.g. densely populated areas and hotspots. Furthermore, pico-cells can also be used to provide coverage in specific locations such as within a residential home or office.
In order to efficiently exploit the additional resource, it is important that handover performance between the macro-cell layer and the underlying layer is optimized. The process of handover can be separated into three phases. Firstly, identifying that a handover might be required, secondly, identifying a suitable handover candidate and finally, switching the mobile user from one base station to another.
The current trend is towards introducing a large number of pico-cells to 3G systems. For example, it is envisaged that residential access points may be deployed having a target coverage area of only a single residential dwelling or house. A widespread introduction of such systems would result in a very large number of small underlay cells within a single macro-cell.
However, underlaying a macro-layer of a 3G network with a pico-cell (or micro-cell) layer creates several issues. For example, the introduction of a large number of underlay cells creates a number of issues related to the identification of individual underlay cells when e.g. handing over to an underlay cell. In particular, 3G communication systems are developed based on each cell having a relatively low number of neighbours and extending the current approach to scenarios wherein the mobile station may need to consider large numbers of potential neighbour cells is not practical.
One problem of extending current approaches to scenarios where there are many underlaying pico-cells is how to detect that a handover is needed and uniquely and efficiently identify the target pico-cell (or microcell). Specifically, it is not practically feasible to assign individual pilot signal scrambling codes to each underlay cell and to identify all potential handover underlay cells as neighbours of the macro-cell as this would require very large neighbour lists. These large neighbour lists would e.g. result in the neighbour list exceeding the maximum allowable number of neighbours in the list, slow mobile station measurement performance as a large number of measurements would need to be made, increased resource usage etc. It would furthermore require significant operations and management resource in order to configure each macro-cell with the large number of neighbours and would complicate network management, planning and optimisation.
However, sharing scrambling codes for the pilot signals of the pico-cells results in a target ambiguity and prevents the mobile station from uniquely identifying a potential handover target. For example, if a group of base stations supporting different underlay cells underlaying a given macro-cell use an identical shared pilot signal scrambling code, a mobile station detecting the presence of this shared scrambling code will be aware that a potential handover target has been detected but will not be able to uniquely identify which of the group of underlay cells has been detected.
Furthermore, a system wherein each underlay cell transmits a pilot signal to support handovers generates a large amount of interference which may significantly degrade the performance of the system. In particular, sharing scrambling codes tend to result in the pilot signals from different underlay cells interfering with each other. Furthermore, in scenarios where remote stations are only allowed to use some underlay cells, a large number of handover attempts to underlay cells which the remote station is not allowed to use may occur. These handover attempts will be rejected but will result in a significant resource usage.
Hence, an improved cellular communication system would be advantageous and in particular a system allowing increased flexibility, improved suitability for large numbers of potential handover targets/neighbour cells, improved suitability for overlay/underlay handovers, reduced neighbour lists, reduced handover attempts, reduced interference, reduced signalling, reduced resource overhead, increased practicality, reduced measurement requirements, facilitated and/or improved handover target detection/identification and/or improved performance would be advantageous.